The present invention relates to a thermostat, in particular for electric household appliances, and which is especially suitable for refrigerators and freezers.
As is known, various types of thermostats for electric household appliances are currently available.
Mechanical thermostats are one type, which employ an expanding, temperature-sensitive mechanical member connected to a switch to turn an actuating device on or off. In refrigerators and freezers, for example, to which the following description refers purely by way of example, the actuating device is defined by a compressor connected to the cooling circuit; the expanding mechanical member, e.g. an expansion box, is connected to a mechanical device for setting the thermostat to a predetermined temperature; and, when the detected temperature differs from the predetermined temperature, the thermostat activates the switch to turn the compressor on or off.
Being easily damaged, mechanical thermostats have the drawback of being unreliable, besides being inaccurate, which is why they have now been replaced with digital and analog electronic or electromechanical types.
Analog thermostats comprise a regulating circuit and employ a sensor (e.g. a thermocouple) for supplying a signal, e.g. a voltage, indicating the value of the temperature to be regulated. Known analog thermostats, however, are also inaccurate as regards the user interface, and call for relatively complex signal processing circuits.
For this reason, users in particular prefer digital thermostats, which sample and compare the sensor signal with a reference value by means of a microprocessor. A nonvolatile (e.g. EEPROM) memory may store the reference values corresponding to each selectable target temperature value.
As compared with analog thermostats, digital types therefore have the advantage of selecting extremely precise reference values for the desired target temperature and so enabling better adjustment. And user information may be provided concerning the currently set target temperature, e.g. by indicating the temperature value on a display.
On the other hand, digital thermostats have the drawback of being extremely expensive, by comprising complex components such as a microprocessor and nonvolatile memory.
It is an object of the present invention to provide a thermostat designed to eliminate the aforementioned drawbacks, and which, in particular, is cheap and easy to produce, while at the same time providing for fairly accurate, reliable temperature adjustment.
It is a further object of the invention to provide a thermostat of the above type, which, in a preferred embodiment, provides a user display of, or at least an ideogram proportional to, the set target temperature, so that, as far as the end user is concerned, the thermostat is xe2x80x9cequivalentxe2x80x9d in performance to a digital type.
According to the present invention, there is provided a thermostat, in particular for electric household appliances, for activating an actuating device, in particular a refrigerator or freezer cooling circuit compressor, and characterized by comprising, in combination: a mechanical switch that selectively assumes a number of different configurations; and an alalog regulating circuit including an input contact receiving an input voltage from a temperature sensor, and a variable-ratio reference divider; said reference divider comprising a reference resistor, and resistive means for selectively assuming a number of different predetermined electric resistance values; said mechanical switch connecting said resistive means to said reference resistor so that, for each configuration in said number of different configurations of said switch, said resistive means selectively assume a predetermined electric resistance value in said reference divider.
In a preferred embodiment, the analog regulating circuit comprises an operational amplifier and a selector. The operational amplifier has a noninverting terminal connected to a reference node of the reference divider, and an inverting terminal connected to said input contact; and the selector has an input terminal connected to a mains supply line, a first output terminal connected to a floating terminal, and a second output terminal connected to an actuating device.
The thermostat is therefore extremely cheap and easy to produce, while at the same time permitting accurate selection of the desired target temperature value and accurate temperature adjustment.
In a preferred variation, the thermostat also comprises a device for displaying the set temperature, and comprising, for example, a battery of LED""s.
This therefore affords the further advantage of a user display indicating the currently set temperature, as on more expensive digital thermostats.